It's part of a combo known as the 'Pickles lock,' wherein Brine Elemental is the 'Pickles' and Vesuvan Shapeshifter is the combo piece that copies Brine Elemental turn after turn, preventing your opponents from untapping
Having a consistent way to not only create a second combat phase but also untap all the creatures you control for a measly four mana is a phenomenal pairing, especially when used as a Commander to bring her back into the fight after being targetted
Bling out your EDH Commander deck, or your collection binder by adding a shadow signature to the front, or a sketch to the backor both
Everyone had a different opinion on how to execute upon our shared vision
Even if it turns out that you cant use him to sacrifice in the same turn, there is still plenty of use to be had if you get a couple of these guys in your graveyard
Doing so does automatically K.O